This is a proposal to continue investigation of the role of serotonin (5-HT) and serotonergic neurotransmission in auditory (A1) cortex excitability following early onset sensorineural hearing loss (SNHL). The PI has been working in this area for the last 2 years and has accumulated interesting pilot data implicating a role for 5-HT, and specifically 5-HT2 receptors, in A1 functioning following bilateral cochlear ablation, as an animal model for early-onset, pre-hearing SNHL. These studies have utilized in vitro whole-cell patch clamp recordings from layer II/III pyramidal cell neurons in rat brain slices that include A1 cortex, following cochlear ablation or sham surgery. Electrophysiologic traces from A1 pyramidal cells have been recorded in the presence of extra-cellular 5-HT application, as well as 5-HT2 receptor antagonists. Furthermore, we have also successfully mapped 5-HT2A receptor protein levels using radio-ligand binding, and measured 5-HT and 5-HT metabolite tissue content using high performance liquid chromatography (HPLC), in A1 during normal postnatal development. In the present proposal, we plan to extend these preliminary studies in two important areas in order to determine: (1) which 5-HT2 receptor subtype(s) mediate serotonergic regulation of A1 pyramidal cell excitability following early postnatal cochlear ablation; and (2) evaluate plasticity within the serotonergic system in A1 in this animal model of early-onset SNHL. Specifically, we plan to use selective 5-HT2A/2B/2C receptor antagonists in patch clamp experiments to evaluate specific 5-HT receptor-based mechanisms responsible for A1 pyramidal cell excitability following cochlear ablation. We also plan to repeat HPLC tissue evaluation of 5- HT and metabolite levels as well as focused 5-HT2 receptor subtype radioligand binding following cochlear ablation as a measure of plasticity within the serotonergic system following early-onset SNHL. This proposal is aimed at developing methods, collecting additional preliminary data, and becoming familiar with a new literature on auditory cortical as well as serotonergic plasticity and organization, function and development, with the ultimate goal of the identification of specific hypotheses that can be pursued in a subsequent R01 application to investigate specific serotonin receptor signal transduction pathways. PUBLIC HEALTH RELEVANCE: The mammalian cerebral cortex is one of the most complex information processing systems known. Although cortical neurons exhibit a variety of intrinsic electrical properties, which are critical for function, it is the synaptic interactions between cells, afferent neurotransmitter systems and the organization of local circuits that play the most critical roles in cortical computation and function. Within the auditory system, the cortex stands in contrast to the medullary and midbrain auditory structures which, although performing significant processing, rely more heavily on the intrinsic electrical properties of neurons embedded in a relatively fixed circuit. Hearing impairments during early postnatal development produce significant changes in the human auditory cortex that may permanently diminish auditory perceptual skills and compromise language acquisition (Psarommatis et al., 2001; Kidd et al., 2002; Emmorey et al., 2003). These affects are particularly severe in children who are born with or who develop an early pre-lingual sensorineural hearing loss; producing long periods of auditory deprivation (Sharma et al., 2002). Profound changes in auditory processing have been observed in hearing- impaired animals (Syka, 2002), and recent data have shown that bilateral cochlear ablation produces heightened excitability of primary auditory (A1) cortical neurons in postnatal rodents (Kotak et al., 2005), suggesting that this experimental paradigm may be useful to investigate mechanisms underlying early sensorineural hearing loss (SNHL). Preliminary studies from our laboratory have investigated the contribution of serotonin (5-hydroxytryptamine; 5-HT), mediated by 5-HT2 receptors, to A1 neuronal excitability in this rodent model of SNHL. In this proposal, we have designed two specific AIMS to test the hypothesis that cochlear ablation, as a model of early onset SNHL, alters the 5-HT2 receptor-linkage of A1 pyramidal cell function and induces compensatory plasticity within the serotonergic system.